https://autocrypt.org/

Autocrypt is a set of guidelines for convenient end-to-end-encryption of e-mails. Developers are working on bringing it to several different e-mail programs.

Delta Chat

https://delta.chat/

Delta Chat is a modern messenger. It is like email in a new dress. Just better, safer and user-optimised.

K-9 Mail

https://k9mail.github.io/

K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email

Enigmail

https://www.enigmail.net

Enigmail is a seamlessly integrated security add-on for Mozilla Thunderbird. It allows you to use OpenPGP to encrypt and digitally sign your emails and to decrypt and verify messages you receive.